1. Field of the Invention
This invention generally relates to optical devices and methods for the manufacture of such optical devices and more particularly to optical devices with lens systems of a small diameter or cross section.
2. Description of Related Art
Endoscopes are examples of optical devices that utilize optical systems characterized by an assembly of a plurality of optical elements, such as lenses, that are serially disposed along an optical axis. In an endoscope, for example, a lens system comprising multiple lens elements at a distal end constitutes an objective; a lens system at the proximal end constitutes an eyepiece; and one or more groups of intermediate lens elements define one or more relay lens systems.
Endoscopes utilizing such systems generally have working channels and lumens. Some working channels are filled with fiber to enable an external light source to illuminate a field of view. Others allow a surgeon to move instruments along the length of the endoscope to perform some function at the distal end while simultaneously viewing the area being treated. Still other working channels allow a surgeon to dispense a therapeutic, diagnostic or other material at the distal end of the endoscope, again while simultaneously viewing the area being treated.
Endoscopes and other optical devices of this nature generally are formed with cylindrical lens elements extending along a centered optical axis. The lens elements generally have concave, planar or convex image forming surfaces that are transverse to the optical axis. Multiple lens elements may be adjoined in lens systems in order to achieve particular optical characteristics, all as known in the prior art. Such lens elements and lens systems are called centered, rotationally symmetrical lens elements and systems, respectively.
Medical personnel who use these optical devices now indicate a preference for optical devices that have smaller and smaller diameters or cross sections. In fact some optical devices are now produced with an outer diameter of 1 to 2 mm using traditional lens making methods. However products that achieve these goals are difficult and expensive to manufacture with traditional lens making methods.
Traditional lens making methods include grinding and polishing operations to produce approximately spherical or other shaped image forming surfaces at the entrance and exit faces that define the optical characteristics of that lens element. Then the lens element is rotated about its geometric axis that will generally lie on the optical axis. A geometric axis is defined as a straight line locus of the centers of curvature of the refracting surfaces. The outer lens boundary then can be made essentially circular, as by abrasive grinding, such that the result is essentially a right circular cylinder with imaging forming spherical end surfaces and a cylindrically centered axis, i.e., a centered, rotationally symmetrical lens element. Individual lens elements can then be adjoined along the coincident optical and geometric axes to form a lens system.
The ability to make smaller optical devices including those with lens systems that continue to exhibit centered rotationally symmetrical characteristics, becomes more difficult as the lens diameter reduces. First, the final diameter of the lens is controlled by the location of the grinding or edging tool with respect to the optical axis including any positional variation due to tolerances in the manufacturing equipment. In conventional lenses these tolerances do not constitute a significant portion of the overall lens diameter. However, to achieve an absolute tolerance as a constant percentage of very small diameters requires extreme accuracy and tools that operate with extremely close tolerances. Machines for providing such accuracies become increasingly expensive as tolerance requirements become more stringent.
Second, in these optical devices, a lens element generally has an axial length that is several times the diameter. At small diameters it becomes difficult to support the lens element so that its optical axis remains in a single position relative to a tool reference. Moreover, as the diameter decreases the lens element becomes, in effect, more brittle and thus extremely fragile. These factors lead to an increased potential for breakage during manufacture.
Thus about 1-2 mm tends to be a practical minimum diameter for any lens element manufactured by traditional lens manufacturing methods. Lens systems in most currently commercially available endoscopes have an outer diameter of approximately 1.7 mm or greater. Endoscopes with such readily available lens elements are too big to be used in many applications including (1) medical applications such as viewing fine vascular structure, (2) minimally invasive endoscopy such as neurological and neurosurgical applications and arthoscopy, ear, nose and throat (ENT) applications, (3) cardiac surgical applications, and (4) endoscope applications that can benefit from the use of stereoscopic endoscopes.
What is needed is a method for enabling the efficient manufacture of high quality lens elements and lens systems having cross sectional dimensions that can be as little as 1 mm or less.